The present invention relates generally to improvements in the electrodeposition of zinc and zinc alloys from aqueous alkaline plating baths and to new additives for use in such electrodeposition processes.
Electrodeposition of zinc and zinc alloys, based for example on sodium zincate, has been known for many years. It is not possible to produce a commercially acceptable deposit from a simple sodium zincate electrolyte as the deposit is powdery and dendritic. For this reason, various additives have been proposed to provide improved deposition, such as cyanides (which have obvious environmental problems) and polymers of amines and epichiorohydrin which act as grain refining additives. These polymers are limited to usage in baths having relatively low concentrations of zinc because it is not possible to prevent uncontrolled deposition of zinc at higher metal concentrations. Also, electroplating processes using these additives tend to have poor cathode efficiency, a narrow bright range, a narrow operating window and tend to produce pitted and “burnt” deposits.
More recently, additives have been proposed which allow higher zinc concentrations to be used, which have significantly reduced burning and pitting and which allow a wider range of operating parameters. Further, the additives enable an excellent deposit distribution (that is, evenness of the deposit across the article being plated, irrespective of its shape in particular areas). This maximises the efficiency of zinc usage. These additives are based generally on polyquaternary amine compounds and are described in U.S. Pat. No. 5,435,898 and U.S. Pat. No. 5,405,523, which also provide further discussion of the prior art.
U.S. Pat. No. 5,435,898 describes polymers for use as additives in the electrodeposition of zinc and zinc alloys, the polymers having the general formula:
where R1 to R4 may be the same or different and are, inter alia, methyl, ethyl or isopropyl and Y may be S or O. R5 is an ether linkage such as (CH2)2—O—(CH2)2.
U.S. Pat. No. 5,405,523 claims ureylene quaternary ammonium polymers in general as brightening agents in zinc alloy electroplating baths. The preferred and exemplified polymers include units of the general formula:
where A may be O, S or N and R may be, inter alia, methyl, ethyl or isopropyl. In the preferred polymers, these units are linked by units derived from, for example a bis(2-haloethyl) ether, a (halomethyl) oxirane or a 2, 2′-(ethylenedioxy)-diethylhalide. Ethylene dihalides such as ethylene dichloride and ethylene dibromide are also suggested but not exemplified.
Further known additives are polycationic compositions based on polymerisation of dimethyl-diallyl ammonium chloride with sulphur dioxide as described in DE 19,509,713.
However, the overall cathodic efficiency of these processes can be low and the resultant deposits may be unsatisfactory in terms of brightness and levelling.
A recent patent application PCT/GB00/00592 provides improved polymers for use as additives in the electrodeposition of zinc and zinc alloys. In particular, this application suggests that by avoiding an ether-type linkage such as R5 in the prior art above, a brighter deposit can be obtained to which it is also easier subsequently to apply conversion coatings.
Another very desirable feature of the electrodeposition of zinc is that the adhesion of the deposit to the substrate must be excellent. This is because zinc has a very low permeability for hydrogen. Thus hydrogen, which is absorbed into the substrate (particularly steel) during the deposition process, may subsequently accumulate at the interface between the substrate and coating and give rise to the formation of “blisters”. Such blistering can occur when using the additives in accordance with the above prior art.
WO 00/14305 describes polymers of the general formula:

The applicants have investigated polymers of this type as plating additives and have found that that the overall cathode efficiency remains low and the resistance of the resulting deposit to post-plate blistering is still relatively poor.